Move the helpers in /usr/libexec now that's allowed in the policy

debian/gnome-control-center.install

 usr/bin
-usr/lib/gnome-control-center
+usr/libexec
 usr/share/applications
 usr/share/bash-completion
 usr/share/dbus-1

debian/rules

@@ -7,9 +7,9 @@ export DEB_LDFLAGS_MAINT_APPEND = -Wl,-O1 -Wl,-z,defs -Wl,--as-needed
 %:
 	dh $@ --with gnome
 
+# -Dgnome_session_libexecdir can be removed after Bullseye release
 override_dh_auto_configure:
 	dh_auto_configure -- \
-		--libexecdir=/usr/lib/gnome-control-center \
 		-Dgnome_session_libexecdir=/usr/lib/gnome-session \
 		-Ddocumentation=true